With reference to FIG. 1A, one type of dipole antenna 20 is a wire dipole comprised of first and second collinear wire/rod elements 22A, 22B that are separated from one another by a gap 24. The first element 22A extends from a first outer end 26A to a first inner end 28A. Similarly, the second element 22B extends from a second outer end 26B to a second inner end 28B. The feed points of the antenna 20 are at or very near the first and second inner ends 28A, 28B.
The dipole antenna 20 exhibits an impedance bandwidth, i.e., a frequency range over which the antenna is effectively operational. The impedance bandwidth of an antenna is typically defined as the ratio of the high frequency (fihigh) to the low frequency (filow) at which the power output of the antenna has a voltage-standing-wave ratio (VSWR) of less than 3.5:1. FIG. 1B illustrates the impedance bandwidth of an embodiment of the antenna 20 being about 1.3:1 (i.e., for a high frequency of 1.7 GHz—and a low frequency of 1.3 GHz). Because the impedance bandwidth of the dipole antenna 20 is less than about 2:1, the dipole antenna 20 is considered to be a narrowband antenna. The length of the dipole antenna 20 is related to filow of the impedance bandwidth. To elaborate, the length of the dipole antenna 20 is the distance between the first outer end 26A and the second outer end 26B. This length is λilow/2, where λilow is the wavelength at filow. The dipole antenna 20 also exhibits a gain bandwidth that is typically defined as the difference between the high frequency (fghigh) and low frequency (fglow) at which the power output of the antenna is greater than 0 dBi. An antenna with a gain bandwidth in which the ratio of the high to low frequencies is less than about 2:1 is also commonly referred to as a narrowband antenna. The dipole antenna 20 is considered to be a narrowband antenna.
With reference to FIG. 2A, a second type of dipole antenna is illustrated, namely, a bowtie dipole antenna 30. The bowtie dipole antenna 30 comprises first and second coplanar triangular-like elements 32A, 32B that are separated from one another by a gap 34. Each of the elements 32A, 32B can either be a flat metal sheet or a wire bent so as to form a triangle like shape. The first element 32A extends from a first outer end 36A to a first inner end 38A. Similarly, the second element 32B extends from a second outer end 36B to a second inner end 38B. The feed points of the antenna 30 are at or very near the first and second inner ends 38A, 38B. Like the dipole antenna 20, the bowtie dipole antenna 30 has an impedance bandwidth. FIG. 2B illustrates that the impedance bandwidth of an embodiment of the antenna 30 is greater than 6:1. In this case, fihigh is at least 6 GHz and filow is about 1 GHz. Hence, the impedance bandwidth is greater than 6:1. The length of the bowtie dipole antenna 30 (i.e., the distance between the first outer end 36A and the second outer end 36B is λilow/2, where λilow is the wavelength at filow. The bowtie dipole antenna 30 is considered to be a broadband antenna, i.e., an antenna with both an impedance bandwidth and gain bandwidth greater than 2:1.
FIG. 4F illustrates the gain bandwidth plots for the embodiment of the cavity-backed dipole antenna and for a reference wire dipole antenna of the same length;